In late years, there has been a need for an optical recording medium capable of affording still higher recoding density for processing a huge amount of information such as dynamic picture image information, and the research and development have been actively conducted for achieving increased recording density and capacity in an optical recording medium.
One example of such achievements is found in DVDs, in which the wavelength of a recoding and reproducing laser beam is made shorter and the numerical aperture (AN) of an objective lens is made larger so that the diameter of a focal spot produced during recording or reproducing is reduced. Actually in DVDs, the recording and reproducing wavelength λ of 780 nm used in CDs is changed to 650 nm in DVDs, and the numerical aperture (NA) of 0.45 used in CDs to 0.6 in DVDs, whereby a recording capacity increased to 6- to 8-fold that of CDs (4.7 GB per side) has been achieved.
However, the increased numerical aperture will raise various problems. For example, the increased numerical aperture decreases the allowance for aberration produced due to an angle by which a disk surface is deviated from the perpendicular to the optical axis of an optical pickup, which is known as a tilt angle. In this relation, an allowance for tilt of an optical recording medium with respect to an optical system, which is known as a tilt margin M, is determined by a numerical aperture NA. In other words, the tilt margin M is proportional to λ/{t×(NA)3} when λ denotes a wavelength of a recording and reproducing laser beam, and t denotes a thickness of a substrate. Therefore, in order to ensure a sufficient tilt margin M, it is required to reduce the thickness t of a substrate.
Accordingly, in DVDs, a sufficient tilt margin is ensured by reducing the substrate thickness to about one half (about 0.6 mm) of a typical thickness of conventional CD substrates (about 1.2 mm).
Recently, for the purpose of making it possible to record a high quality dynamic picture image for long hours, a system has been developed to achieve a large recording capacity (larger than 20 GB per side) corresponding to more than 4-fold that of DVDs by decreasing the wavelength λ of a recording and reproducing laser beam to about 400 nm and increasing the numerical aperture (NA) to 0.85.
The optical disk used in this system has a light transmitting layer with a thickness of about 0.1 mm in order to ensure the tilt margin, through which a laser beam for recording or reproducing is launched thereinto.
Moreover, in response to the request for further increase in recording capacity, there is proposed, for example, a so-called single sided dual layer disk that has, on its supporting substrate, a second recording layer (Layer 1), a space layer, a first recording layer (Layer 0), and a light transmitting layer and is used so that a laser beam for recording or reproducing enters through the light transmitting layer. The single sided dual layer disk is disclosed, for example, in JP-A-2000-11453.